1. Field of the Invention
The present invention relates to a control system generating a control signal to a device to be controlled, and in particular, relates to an embedded control system provided with a processor for performing floating-point operations.
2. Description of the Related Art
In recent years, with emissions/fuel efficiency regulations tightened, demanded driving stability/safety levels are increasing. Thus, improved control accuracy is also demanded of embedded control systems of automobiles and adoption of a control unit provided with a processor (FPU) or microcomputer to perform floating-point operations has begun.
In an embedded control system of an automobile, an interpolation search of a table/map is performed from map/table/axis data. The map/table/axis data means map/axis data or table/axis data. The map/axis data is a three-dimensional map with a parameter called the axis set as the X axis and controlled amounts each set as the Y axis and the Z axis. The table/axis data is a two-dimensional table with a parameter called the axis set as the X axis and a controlled amount as the Y axis.
Such data takes discrete values, which are also called reference values. Thus, processing to obtain data with higher precision from such discrete value data is an interpolation search. Data obtained from an interpolation search is used to control a control target.
Map/table/axis data is represented as fixed-point numbers. However, when a floating-point operation processor (FPU) or microcomputer is used, map/table/axis data needs to be represented as floating-point numbers.
The data size of a fixed-point number is 1 byte, 2 bytes, or 4 bytes. The data size of a floating-point number, on the other hand, is 4 bytes for single-precision floating-point number and 8 bytes for a double-precision floating-point number. Thus, the amount of data used for floating-point operations will be approximately double compared with that used for conventional fixed-point operations and the storage area of a processor (FPU) or microcomputer is starting to run short.
When map/table/axis data is stored in a memory using a technology disclosed by Japanese Patent Application Laid-Open No. 2004-225668, the data is defined by fixed-point numbers requiring a smaller data capacity. When an interpolation search of the table/map from map/table/axis data is performed, data in the fixed-point format is converted into data in the floating-point format. Thus, a shortage of storage capacity in the memory can be avoided while at the same time, arithmetic precision is ensured.
When map/table/axis data is converted by conventional technology, physical value conversion data (LSB data and offset data) is used. That is, when an interpolation search is performed, fixed-point map/table/axis data is converted into floating-point format data and multiplied by LSB data, which is physical value conversion data, before offset data being further added thereto. When map/table/axis data is stored in a backup memory, floating-point map/table/axis data is temporarily fetched from a random access memory. Next, the data is limited by a fixed-point maximum value and minimum value. Then, the offset data is subtracted from the data and the resultant data is divided by the LSB data. The map/table/axis data obtained in this way is stored in the backup memory. Thus, there is a possibility that computational load increases.
The present invention has been developed in view of the above subject and an object thereof is to provide an embedded control system capable of ensuring precision in arithmetic with data in the floating-point format and also avoiding a shortage of storage area in a memory.